This invention relates to a method of examining an exposure tool, and more particularly to a method of examining an exposure tool used to measure not only the shape of the light source and that of the pupil of the projection optical system but also the alignment of the axis of the illumination optical system with that of the projection optical system in the exposure tool.
The manufacture of semiconductor-device circuit patterns generally requires lithography. In a projection exposure tool used in lithography, the light emitted from the light source enters the illumination optical system, which illuminates the reticle at almost uniform illuminance. The light passed through the reticle goes through the projection optical system, exposes the photoresist, and forms the on-reticle circuit pattern on the photoresist.
As the area exposed simultaneously by an exposure tool increases, the formation of a pattern differs from place to place in the exposure area. Namely, the dimensions of the pattern vary in the die. One cause of the problem is variations in the shape and light intensity of coherence factor (.sigma.) in the simultaneous exposure area. The coherence factor (.sigma.) represents the effective size of the illumination optical system. In this case, the intensity at which light illuminates the reticle and the direction in which the light incidents the reticle differs place to place. Since .sigma. is a parameter that controls the contrast of an image, variations in .sigma. means variations in the exposure characteristic in the simultaneous exposure area.
The problem of a conventional exposure tool will be explained by reference to FIGS. 1A and 1B. As shown in FIG. 1A, a general exposure tool has a common axis optical system in which an illumination optical system 1, a reticle 2, a projection optical system 3, and a wafer 5 are arranged in a straight line. This arrangement is limited to the design stage. Actually, however, each lens may have deviated from a reference axis in a different direction. If the illumination optical system 1' and the projection optical system 3 do not have common axis, the diffracted light passed through the reticle 2 enters the projection optical system 3 obliquely as a whole. In this case, the image whose position shouldn't be changed even after the defocusing of the position of the wafer 5 to the position 5' as shown in FIG. 1A moves as a result of defocusing as shown in FIG. 1B.
The variation of .sigma. and the light intensity in the simultaneous exposure area and the disagreement between the axis of the illumination optical system and that of the projection optical system combine to narrow the allowance for exposure and focal depth necessary for creating a correct pattern. This makes it difficult to form a very small circuit pattern by lithography, which leads to a decrease in the yield in manufacturing semiconductor devices. To avoid this, it is necessary to examine and adjust not only the .sigma. of the exposure tool but also the deviation of the optical axis of the illumination optical system from that of the projection optical system or vice versa.
If the exposure tool were disassembled and a measuring unit, such as an interferometer or a camera, were provided, measurements could be made with high accuracy. This approach has a problem: the disassembling of the exposure tool would make the state of the tool different from that in the preceding operation. Another problem is to require a lot of time and labor. To overcome these problem, a simple examining method that can be carried out without disassembling the exposure tool is required.
A method of examining an exposure tool without disassembling it, which was carried out by Progler, et al., will be explained by reference to FIGS. 2A to 2C. First, two types of pattern are formed on the back of a reticle by a first step shown in FIG. 2A and a second step shown in FIG. 2B. These patterns are formed on the same reticle or separate reticles. The pattern of FIG. 2A is a pattern in which a transmitting area (an isolated pinhole 121) is isolated in a shading area. The pattern of FIG. 2B is a pattern in which a shading area (an isolated shading dot 122) is isolated in a transmitting area.
The larger the size of the isolated pinhole 121, the greater the diffraction angle of light 123 passing through the pinhole 121. Making use of this phenomenon, the size of the pinhole 121 is so adjusted that the diffracted light illuminates all the surface including the outer edge of the pupil 4 of the projection optical system. The size of the isolated shading dot 122 is made a little large to the extent that the diffraction becomes inconspicuous. First, the isolated pinhole 121 is exposed. The light diffracted at the isolated pinhole 121 illuminates the whole surface of the pupil 4 of the projection system lens. The light passed through the pupil 4 forms an exposure area 125. Next, the reticle 2 is so manipulated without moving the wafer that the isolated shading dot 122 is put in the position where the isolated hole was just before. Then, exposure is made. The pattern is such that the transmitting area is replaced with the shading area in the isolated pinhole 121 of FIG. 2A. In the image transferred to the wafer 5, too, the bright area is reversed into a dark area and the dark area is reversed into a bright area. Because the shading area is so great that it is influenced slightly by diffraction, an image 128 (an image representing the size .sigma. of the light source) appears clearly on the wafer 5. At this time, the amount of exposure is minimized so that the photoresist on the patternless area may be developed on the reticle 2 to such a degree that the development is stopped in a thinner film state. After the double exposure, development is made to produce a photoresist pattern of the shape shown in FIG. 2C. This makes it possible to measure the shape 129 of the light source, the shape 130 of the pupil of the projection optical system, and the difference in position between the shape 129 of the light source and the shape 30 of the pupil.
Their method, however, has the following problems.
Firstly, in the first of the double exposure, diffracted light illuminating the whole pupil of the projection optical system is generated using a single pinhole. To cause this phenomenon, the size of the pinhole has to be made very small. For example when using KrF eximer laser exposure tool (NA=0.6, M=4, and .sigma.=0.75), the diameter of the pinhole must be 4 .mu.m or less to achieve the situation.
When an attempt is made to expose the pinhole of this size existing in the back of the reticle, it takes a very long time to obtain the desired pattern because the light passing through the pinhole and reaching the wafer spreads and becomes is very weak. Making the size of the pinhole larger increases the total dosage of light passing through the pinhole. This, however, decreases the light intensity at the edge of the pupil of the projection optical system, failing to achieve the object.
Secondly, to observe film reduction, it is necessary to recognize a slight difference in contrast. Therefore, judging the shape of an image representing the size of the pupil requires an image processing on a computer to enhance the contrast and observe the photoresist image.
Thirdly, there arises an error in alignment during double exposure. The measured value of the positional deviation includes the error.
Fourthly, since the decreased film state has to be produced in both the first exposure and the second exposure, the adjustment of the dosage of exposure is very delicate.
As described above, although the conventional examining method has solved the problem of measuring an exposure tool without disassembling it, it is hard to say the method is simple.